runtime: avoid infinite loop in growslice

On 386 the below code triggered an infinite loop in growslice:
x = make([]byte, 1<<30-1, 1<<30-1)
x = append(x, x...)

Check for overflow when calculating the new slice capacity
and set the new capacity to the requested capacity when an overflow
is detected to avoid an infinite loop.

No automatic test added due to requiring to allocate 1GB of memory
on a 32bit plaform before use of append is able to trigger the
overflow check.

Fixes #21441

Change-Id: Ia871cc9f88479dacf2c7044531b233f83d2fcedf
Reviewed-on: https://go-review.googlesource.com/57950
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/slice.go

@@ -105,9 +105,16 @@ func growslice(et *_type, old slice, cap int) slice {
 		if old.len < 1024 {
 			newcap = doublecap
 		} else {
-			for newcap < cap {
+			// Check 0 < newcap to detect overflow
+			// and prevent an infinite loop.
+			for 0 < newcap && newcap < cap {
 				newcap += newcap / 4
 			}
+			// Set newcap to the requested cap when
+			// the newcap calculation overflowed.
+			if newcap <= 0 {
+				newcap = cap
+			}
 		}
 	}